Vulcanizable flouroelastomer composition

ABSTRACT

A vulcanizable fluorine-containing elastomer composition having excellent stability in molding and kneading property, from which a vulcanized article having a small compression set can be produced in a short period of time. The vulcanizable fluorine-containing elastomer composition comprises a vulcanization agent, a vulcanization aid and a fluorine-containing elastomer having C—H bond in its polymer chain, and an optimum vulcanization time T 90  (min) of the composition and a compression set CS (%) of the vulcanized article satisfy the following equations (I) and (II):
     Equation (I): in case of 50,000≦MW≦200,000,
 
 CS ≦−3.45 Ln( T   90 )+22.8−4×10 −5   ×MW  
   Equation (II): in case of 200,000&lt;MW,
 
 CS ≦−3.45 Ln( T   90 )+14.8.

TECHNICAL FIELD

The present invention relates to a novel fluorine-containing elastomerwhich has few branched chains and undergoes a small change in its weightat high temperatures (reduced thermal deterioration and evaporation) andrelates to a process for preparation of the elastomer. Also, the presentinvention relates to an elastomer composition which exhibits excellentflowability at processing, can be vulcanized effectively, does notcontaminate a metal die, has excellent mold-releasing property andprovides a fluorine-containing vulcanized article which possesses asmall compression set and excellent mechanical properties.

The vulcanized article of the present invention can be used suitablyparticularly for diaphragm and sealing materials such as O-ring, oilseal, packing and gasket.

BACKGROUND ART

Vinylidene fluoride (VdF) elastomers comprising various components havebeen proposed so far. Examples of such a known VdF elastomer are acopolymer of VdF and hexafluoropropylene (HFP) (VdF/HFP=60 to 15/40 to85 in % by weight) (for example, JP-B-33-7394); a terpolymer oftetrafluoroethylene (TFE), VdF and HFP in which an amount of TFE is from3 to 35% by weight and a weight ratio of VdF/HFP is from 2.33/1 to0.667/1 (for example, JP-B-36-3495); a terpolymer of tetrafluoroethylene(TFE), VdF and HFP in which an amount of TFE is from 10 to 30% by weightand a weight ratio of VdF/HFP is from 1.6/1.0 to 4.0/1.0 (for example,JP-B-48-18957); a terpolymer comprising from 57 to 61% by weight of VdF,from 27 to 31% by weight of HFP and from 10 to 14% by weight of TFE (forexample, JP-A-53-149291); and the like. Further there was proposed aprocess for preparing, in the presence of a chain transfer agent, a VdFelastomer comprising VdF and at least one other fluorinatedethylenically to unsaturated monomer having carbon atoms and at leastthe same number of fluorine atoms (JP-A-47-5944).

However the elastomer of the present invention explained hereinbelow hasnot been obtained by any of those means. This is because those patentpublications take uniformity of a polymer structure into considerationand are silent with respect to setting of reaction conditions directedto a reaction to produce branch chains and thus even concrete means tosolve problems are not suggested.

Fluorine-containing elastomers are molded by means of usual rubberprocessing equipment. Namely, the elastomers are processed in the orderof kneading with a kneading roll or kneader, molding with an extruder,calendar roll or press, primary vulcanizing by press injection and thenfinally secondary vulcanizing with an oven.

However only with the above-mentioned fluorine-containing elastomers,there are problems with mold-processing, namely processability with aroll at kneading and mold-releasing property at compression molding areinferior, contamination of a metal die cannot be fully prevented andflowability at injection molding is not good. Therefore in order tosolve those problems with mold-processing, widening of a molecularweight distribution has been proposed (for example, JP-A-52-62391,JP-A-4-209643, etc.).

On the contrary, there arose a tendency that by increasing a content oflow molecular weight components, resistance against compression set waslowered, and mechanical strength such as elongation and tensile strengthand further solvent resistance were lowered.

As mentioned above, physical properties such as resistance againstcompression set and mechanical properties are considered contrary tomold-processability. Actually in order to take a balance of physicalproperties and mold-processability, a molecular weight and a molecularweight distribution of a polymer are adjusted, a structure of thepolymer end is changed or any of properties are sacrificed depending onapplications.

Further with respect to the fluorine-containing elastomer compositionfor a sealing material such as O-ring and gasket, a particularly lowcompression set and further an efficient vulcanization (highvulcanization rate and high crosslinking density) are required.Compression set is an important factor for evaluating a sealingperformance, and if the compression set is high, sealing performance islost in a short period of time. Vulcanizability is a factor greatlyinfluencing productivity in mold-processing, and it is desired that thevulcanizability is as high as possible within the range satisfyingrequired physical properties. Also particularly in injection molding, anexcellent flowability is required. If the flowability is inferior,pouring into a metal die is difficult and a molding material must beused in an amount more than required. Also there is a case where athickness of a molded article becomes non-uniform.

In order to make vulcanization efficient, there is proposed that thenumber of ionic ends (or acid ends) attributable to ammonium persulfatewhich has been used as a polymerization initiator is minimized as low aspossible. For example, there are a method of using an oil solubleorganic peroxide as a polymerization initiator (for example,JP-A-6-302487, JP-A-8-301940) and a method of using a fluoroalkylsulfinate and organic peroxide together (U.S. Pat. No. 5,256,745).However in those methods, processability other than vulcanizabilitycannot be satisfied.

To enhance the both of vulcanization efficiency and resistance againstcompression set simultaneously, there is proposed a method of adding aphosphonate compound or the like as a vulcanization accelerator(JP-A-62-54750). However even in this method, there remains a problemthat enhancement of processability cannot be attained.

Further though the flowability is intended to be improved by decreasinga molecular weight as mentioned above, resistance against compressionset and vulcanization efficiency are still insufficient.

JP-A-10-130447 discloses a technique that by vulcanizing in an oven for0 to 30 minutes, a compression set becomes not more than 20%, a thermalstability is high and a content of a low molecular weight componenthaving a molecular weight of not more than 10,000 is small. Howeveraccording to concrete preparation examples of the publication, even ifthe vulcanization in an oven is carried out for not less than 30minutes, compression set does not change as compared with that of30-minute vulcanization in an oven and is only about 14%. Namely,reduction of a content of a low molecular weight component is not madesufficiently and is still insufficient from the viewpoint of recentmarket demand.

An object of the present invention is to provide a novelfluorine-containing elastomer which has few branched chains andundergoes a small change in its weight at high temperatures (reducedthermal deterioration and evaporation) and to provide a process forpreparation of the elastomer.

Another object of the present invention is to provide afluorine-containing elastomer composition which has a high vulcanizationefficiency and excellent mold-processability and provides a vulcanizedarticle which undergoes a small compression set.

DISCLOSURE OF INVENTION

Namely, the present invention relates to a fluorine-containing elastomercomprising vinylidene fluoride (VdF) unit and hexafluoropropylene (HFP)unit and characterized in that:

-   (1) when a 0.2% by weight acetone solution of the    fluorine-containing elastomer is subjected to pressurized filtration    under a condition where acetone passes through at a flow rate F₀ of    2.4 g/min·cm², a ratio F₀/F₁ is from 1.2 to 3.5, preferably from 1.2    to 3.0, provided that F₁ is an amount of filtrate of the acetone    solution during initial one minute, and-   (2) a weight average molecular weight MW measured by GPC is from    50,000 to 600,000.

Preferably the above-mentioned fluorine-containing elastomer is furthercharacterized in that:

-   (3) a ratio of an average molecular weight MW_(LS) measured by    GPC-LALLS to MW_(RI) measured by GPC-RI, namely a ratio    MW_(LS)/MW_(RI) is from 1 to 2.5.

The present invention relates to a fluorine-containing elastomer whichcomprises VdF unit and HFP unit and is characterized in that a weightaverage molecular weight MW measured by GPC is from 50,000 to 600,000,and when the elastomer is vulcanized in the following standardformulation under the following standard vulcanization conditions, anoptimum vulcanization time T₉₀ (min) and a compression set CS (%)satisfy the following equations (I) and (II):

-   Equation (I): in case of 50,000≦MW≦200,000,    CS≦−3.45 Ln(T ₉₀)+22.8−4×10⁻⁵ ×MW-   Equation (II): in case of 200,000<MW,    CS≦−3.45 Ln(T ₉₀)+14.8.    Standard Formulation

100 parts by weight of a fluorine-containing elastomer, 2.2 parts byweight of bisphenol AF, 0.4 part by weight of benzyltriphenylphosphoniumchloride, 3 parts by weight of a highly activated magnesium oxide, 20parts by weight of a carbon black MT-C and 6 parts by weight of calciumhydroxide.

Standard Vulcanization Conditions

-   -   Kneading method: Kneading by roll    -   Press vulcanization: 170° C. for 10 minutes    -   Vulcanization in an oven: 230° C. for 24 hours.

It is preferable that the fluorine-containing elastomer comprises 50 to90% by mole of VdF unit and 50 to 10% by mole of HFP unit and containsionic end groups in an amount of from 10⁻⁵ to 10⁻² moles per 1 kg of theelastomer and further a weight reduction ratio of the elastomer whenheated at 250° C. for 48 hours is not more than 1% by weight.

It is preferable that the fluorine-containing elastomer satisfies thefollowing equation (IV):Y≦−5.3 Ln(X)+28,provided that X is a Mooney viscosity (1+10) at 100° C. and Y is apercent by weight of a low molecular weight component having a weightaverage molecular weight measured by GPC of not more than 30,000 in thefluorine-containing elastomer.

The present invention also relates to a vulcanizable fluorine-containingelastomer composition which comprises a vulcanization agent, avulcanization aid and a fluorine-containing elastomer having C—H bond inits polymer chain and characterized in that:

-   (1) when a 0.2% by weight acetone solution of the    fluorine-containing elastomer is subjected to pressurized filtration    under a condition where acetone passes through at a flow rate F₀ of    2.4 g/min·cm ², a ratio F₀/F₁ is from 1.2 to 3.5, provided that F₁    is an amount of filtrate of the acetone solution during initial one    minute, and-   (2) a weight average molecular weight MW measured by GPC is from    50,000 to 600,000.

Preferably the above-mentioned fluorine-containing elastomer is furthercharacterized in that:

-   (3) a ratio of an average molecular weight MW_(LS) measured by    GPC-LALLS to MW_(RI) measured by GPC-RI, namely a ratio    MW_(LS)/MW_(RI) is from 1 to 2.5.

The present invention relates to a vulcanizable fluorine-containingelastomer composition which comprises a vulcanization agent, avulcanization aid and a fluorine-containing elastomer having C—H bond inits polymer chain and characterized in that a weight average molecularweight MW measured by GPC of the elastomer is from 50,000 to 600,000 andwhen the elastomer is vulcanized in the above-mentioned standardformulation under the above-mentioned standard vulcanization conditions,an optimum vulcanization time T₉₀ (min) and compression set CS (%)thereof satisfy the following equations (I) and (II):

-   Equation (I): in case of 50,000≦MW≦200,000,    CS≦−3.45 Ln(T ₉₀)+22.8−4×10⁻⁵ ×MW-   Equation (II): in case of 200,000<MW,    CS≦−3.45 Ln(T ₉₀)+14.8.

Further the present invention relates to a vulcanizablefluorine-containing elastomer composition which comprises avulcanization agent, a vulcanization aid and a fluorine-containingelastomer having C—H bond in its polymer chain and is characterized inthat an optimum vulcanization time T₉₀ (min) of the composition and acompression set CS (%) of a vulcanized article satisfy the followingequations (I) and (II):

-   Equation (I): in case of 50,000≦MW≦200,000,    CS≦−3.45 Ln(T ₉₀)+22.8−4×10⁻⁵ ×MW-   Equation (II): in case of 200,000<MW,    CS≦−3.45 Ln(T ₉₀)+14.8.

In those compositions, it is preferable that a copolymer of vinylidenefluoride and other monomer is used as the fluorine-containing elastomerand at least one selected from the group consisting ofhexafluoropropylene, tetrafluoroethylene and perfluoro(alkyl vinylether) is used as the other monomer.

Example of the vulcanization agent is preferably at least one selectedfrom the group consisting of polyhydroxy compounds, polyamine compoundsand organic peroxides. Further example of the polyhydroxy compound ispreferably at least one selected from the group consisting of bisphenolAF, hydroquinone, bisphenol A and diaminobisphenol AF. Example of theorganic peroxide is preferably at least one selected from the groupconsisting of α,α′-bis(t-butylperoxy)diisopropylbenzene,2,5-dimethyl-2,5-di(t-butylperoxy)hexane and dicumyl peroxide. Examplesof the polyamine compound are preferably hexamethylenediamine carbamateand/or N,N′-dicinnamylidene-1,6-hexamethylenediamine.

The suitable vulcanization aid is at least one organic base residueselected from the group consisting of quaternary ammonium salts,quaternary phosphonium salts, cyclic amines and mono-functional aminecompounds.

The present invention further relates to a process for preparing afluorine-containing elastomer having C—H bond in its polymer chain byemulsion-polymerizing polymerizable monomers in an aqueous medium in areactor, in which the polymerization is carried out under the conditionsthat:

-   (a) a reaction system in the reactor has a gas phase and a liquid    phase, and-   (b) during the polymerization reaction, a content Cp of the    polymerizable monomers in the polymer particles which are in the    process of polymerization reaction is maintained at a reference    content C₁ or more of the polymerizable monomers in the polymer    particles in which the reference content C₁ is defined as follows.

A pressure resistant reactor is charged with pure water and polymerparticles having the same components and proportion as an intendedpolymer in an amount of 1% by weight based on water and then the gasphase is replaced with nitrogen gas. After introducing a polymerizablemonomer or a monomer mixture corresponding to intended components andproportion under pressure into the gas phase in a concentration of 1.0mole/liter (the concentration is a value obtained by dividing the totalnumber of moles of the introduced monomer by a volume inside the reactorexcluding a volume of the liquid phase), the liquid phase is stirredsufficiently while maintaining the inside of the reactor at a givenreaction temperature. A content of the polymerizable monomers in thepolymer particles when a content of the polymerizable monomers in theliquid phase and the polymer particles comes to equilibrium is definedas the reference content C₁ of the polymerizable monomers in the polymerparticles.

It is particularly preferable that the content Cp of the polymerizablemonomers in the polymer particles which are in the process ofpolymerization reaction is maintained at a reference content C₂ or moreof the polymerizable monomers in the polymer particles in which thereference content C₂ is defined as follows.

A pressure resistant reactor is charged with pure water and polymerparticles having the same components and proportion as an intendedpolymer in an amount of 1% by weight based on water and the gas phase isreplaced with nitrogen gas. After introducing a polymerizable monomer ora monomer mixture corresponding to intended components and proportionunder pressure into the gas phase in a concentration of 2.0 mole/liter(the concentration is a value obtained by dividing the total number ofmoles of the introduced monomer by a volume inside the reactor excludinga volume of the liquid phase), the liquid phase is stirred sufficientlywhile maintaining the inside of the reactor at a given reactiontemperature. A content of the polymerization monomers in the polymerparticles when a content of the polymerizable monomers in the liquidphase and the polymer particles comes to equilibrium is defined as thereference content C₂ of the polymerizable monomers in the polymerparticles.

The polymerizable monomer may be introduced into the reactorcontinuously or batchwise. A water soluble radical polymerizationinitiator is suitable as the polymerization initiator.

The polymerizable monomer which can be used on the above-mentionedprocess is not limited particularly, and the use of vinylidene fluorideand other monomer is advantageous. It is preferable that the othermonomer is at least one selected from the group consisting ofhexafluoropropylene, tetrafluoroethylene and perfluoro(alkyl vinylether).

BEST MODE FOR CARRYING OUT THE INVENTION

First, the fluorine-containing elastomer composition having a highvulcanization efficiency and excellent mold-processability and providinga vulcanized article having a small compression set is explained below.

The fluorine-containing elastomer composition is a vulcanizablefluorine-containing elastomer composition which comprises avulcanization agent, a vulcanization aid and the fluorine-containingelastomer having C—H bond in its polymer chain and is characterized inthat an optimum vulcanization time T₉₀ (min) of the composition and acompression set CS (%) of a vulcanized article satisfy the followingequations (I) and (II):

-   Equation (I): in case of 50,000≦MW≦200,000,    CS≦−3.45 Ln(T ₉₀)+22.84×10⁻⁵ ×MW-   Equation (II): in case of 200,000<MW,    CS≦−3.45 Ln(T ₉₀)+14.8.

Those equations represent a proper relation between vulcanizing time andcompression set and indicate that particularly when the MW is not morethan 200,000, the compression set is influenced also by the MW. On theother hand, the equations indicate that the compression set CS (%) of avulcanized article obtained from the fluorine-containing elastomercomposition of the present invention is small as compared with otherelastomer even in case of the same optimum vulcanization time T₉₀ (min)and weight average molecular weight. The composition satisfying thoseequations has good vulcanizability and processability and provides avulcanized article having a small compression set.

As the molecular weight increases, the compression set tends to belowered. Therefore when the weight average molecular weight is not lessthan 300,000, it is further preferable that the fluorine-containingelastomer composition satisfies the following equation (III).CS≦−3.45 Ln(T ₉₀)+13.3

Examples of the fluorine-containing elastomer having C—H bond in itspolymer chain are, for instance, a copolymer comprising vinylidenefluoride (VdF) unit and other monomer unit, a copolymer comprisingethylene unit and other fluorine-containing monomer unit, and the like.

Then explained below are monomer components of the VdF elastomer whichare particularly suitable for the composition of the present invention.

It is preferable that the other fluorine-containing monomer which iscopolymerized with VdF is a monomer selected from the group consistingof perfluoro olefins and perfluoro(vinyl ethers).

Examples of the perfluoro olefin are, for instance, monomers representedby the formula (I):CF₂═CF—Rf¹  (I)wherein Rf¹ is fluorine atom or a perfluoroalkyl group having 1 to 9carbon atoms. Examples thereof are TFE, HFP, and the like.

Examples of the perfluoro(vinyl ether) are, for instance,perfluoro(alkyl vinyl ether) (PAVE) represented by the formula (II)CF₂═CF—O—Rf²  (II)wherein Rf² is a perfluoroalkyl group having 1 to 5 carbon atoms andperfluoro(alkoxy vinyl ether) represented by the formula (III):CF₂═CF—O—(CF₂CFXO)_(m)—Rf²  (III)wherein Rf² is as defined above, X is fluorine atom or CF₃, m is aninteger of from 1 to 6.

Examples of the PAVE of the formula (II) are, for instance,perfluoro(methyl vinyl ether) (PMVE), perfluoro(ethyl vinyl ether)(PEVE), perfluoro(propyl vinyl ether) (PPVE), and the like.

Examples of the perfluoro(alkoxy vinyl ether) of the formula (III) are,for instance, CF₂═CF—O—(CF₂CF(CF₃)O)₂—CF₂CF₂CF₃, and the like.

In the monomers of the formulae (I) to (III), any of fluorine atoms atends of Rf¹ and Rf² may be carboxyl group, sulfonic acid group, hydroxylgroup or a group derived therefrom.

The VdF elastomers which can be preferably used for the composition ofthe present invention are roughly classified into copolymers andterpolymers. It is a matter of course that the novel fluorine-containingelastomer explained hereinbelow can be used suitably.

In case of the copolymer, a copolymer of VdF and HFP, a copolymer of VdFand CTFE and a copolymer of VdF and PAVE are preferred. Particularlypreferred is the copolymer of VdF and HFP in a mole ratio of from 9/1 to5/5, further preferably from 8.5/1.5 to 6/4.

In case of the terpolymer, a terpolymer of VdF, HFP and TFE and aterpolymer of VdF, TFE and perfluoro(vinyl ether) are preferred.Particularly preferred are the VdF/HFP/TFE terpolymer in which a moleratio of VdF/HFP is from 8.5/1.5 to 6/4 and an amount of TFE is from 10to 30% by mole and the VdF/TFE/perfluoro(vinyl ether) terpolymer inwhich an amount of perfluoro(vinyl ether) unit is from 10 to 50% bymole, further preferably from 30 to 50% by mole and a mole ratio ofVdF/TFE is from 1/9 to 9/1. Preferred perfluoro(vinyl ether) is PAVE.

It is preferable that the fluorine-containing elastomer having C—H bondin its polymer chain is prepared, for example, by the process mentionedbelow since the elastomer having few branched chains and excellentvulcanizability and processability can be obtained.

Namely, the polymerization is carried out by the process for preparingthe fluorine-containing elastomer having C—H bond in its polymer chainby emulsion-polymerizing polymerizable monomers in an aqueous medium ina reactor under the conditions that:

-   (a) a reaction system in the reactor has a gas phase and a liquid    phase, and-   (b) during the polymerization reaction, the content Cp of the    polymerizable monomers in the polymer particles which are in the    process of polymerization reaction is maintained at the reference    content C₁ or more or the reference content C₂ or more of the    polymerizable monomer in the polymer particles which was defined    above.

The reference content C₁ of the polymerizable monomers in the polymerparticles means a content of monomers in the polymer particles in anequilibrium state between the gas phase, liquid phase and polymerparticles in case where the monomer content in the gas phase is 1.0mole/liter. The reference content C₁ is a minimum reference contentinhibiting formation of branched or gelled polymer to an extent notaffecting the physical properties of the elastomer such as compressionset (CS).

The reference content C₂ of the polymerizable monomer in the polymerparticles means a content of monomers in the polymer particles in anequilibrium state between the gas phase, liquid phase and polymerparticles in case where the monomer content in the gas phase is 2.0mole/liter. The reference content C₂ is a minimum reference contentinhibiting formation of branched or gelled polymer to an extent notaffecting the physical properties of the elastomer such as compressionset (CS).

As mentioned above, the polymerizable monomer content in the gas phaseis a value obtained by dividing the total number of moles of theintroduced monomers by a volume inside the reactor excluding a volume ofthe liquid phase and contains the liquefied monomer as far as it is notpresent in the liquid phase and in the polymer particles.

The preparation process is concretely explained below.

Since the polymerization is carried out usually under pressure, thepressure resistant reactor is used. An aqueous medium (usually purewater) containing polymer particles for emulsion polymerizationcomprising the same components and proportion as an intended polymer isput in the reaction tank to form a liquid phase. The inside of thereaction tank comprises this liquid phase and a gas phase. After the gasphase is replaced with nitrogen gas, a polymerizable monomer isintroduced. Then the inside of the reaction tank, particularly theliquid phase is subjected to stirring to supply the polymerizablemonomer from the gas phase to the liquid phase. With the monomersupplied to the liquid phase the polymer particles are impregnated andthus a polymerizable monomer content in the polymer particles isincreased. When supplying of the monomer to the gas phase is continued,since the monomer content in the polymer particles is saturated (it canbe said that a rate of supplying the monomer to the liquid phase comesto an equilibrium), a polymerization initiator is introduced to startpolymerization.

As the polymerization is continued, the monomer is consumed and amonomer content in the produced polymer particles (called “Cp” in thepresent invention) is decreased. A feature of the preparation process ofthe present invention is to maintain this monomer content Cp in theproduced polymer particles at the reference monomer content C₁ or morein the polymer particles. Namely, the polymerization is carried outunder the condition that the monomer can be continuously supplied to thepolymer at a high monomer content. When the polymerization is carriedout under that condition, a uniform polymerization reaction and a highpolymerization rate can be obtained. Even if this condition isdiscontinued for a short period of time, there arises no problem as faras the polymerization reaction is carried out substantially uniformly.

The reference monomer contents C₁ and C₂ are the monomer contents in thepolymer particles and cannot be measured directly. As a factor relatingto the monomer content in the polymer particles, there is apolymerization rate which can be calculated from actually measurablefigures.

For example, a polymerization rate K₁ in the polymer particles(hereinafter referred to as “reference polymerization rate” in apolymerization system maintained at the reference monomer content C₁becomes 0.07 kg/liter·hr under the following polymerization conditions.

(Polymerization Conditions)

-   Content of monomer mixture in gas phase: 1 mole/liter-   Monomer components in gas phase: VdF/HFP=45/55% by mole-   Polymerization temperature: 80° C.-   Content of polymerization initiator (APS): 300 ppm (based on water)-   Content of molecular weight controller (diethyl malonate): 900 ppm    (based on water)-   Number of seed particles: 3×10¹⁴ per 1 ml of water-   Content of emulsifying agent (ammonium perfluorooctanoate): 2,000    ppm (based on water)-   Components of monomer mixture added continuously: VdF/HFP=78/22% by    mole-   Polymerization time: 120 minutes.

Also a reference polymerization rate K₂ in the polymer particles in apolymerization system maintained at the reference monomer content C₂becomes 0.17 kg/liter-hr under the following polymerization conditions.

(Polymerization Conditions)

-   Content of monomer mixture in gas phase: 2 mole/liter-   Monomer components in gas phase: VdF/HFP-43/57% by mole-   Polymerization temperature: 80° C.-   Content of polymerization initiator (APS): 300 ppm (based on water)-   Content of molecular weight controller (diethyl malonate): 2,100 ppm    (based on water)-   Number of seed particles: 3×10¹⁴ per 1 ml of water-   Content of emulsifying agent (ammonium perfluorooctanoate): 2,000    ppm (based on water)-   Components of monomer mixture added continuously: VdF/HFP=78/22% by    mole-   Polymerization time: 240 minutes.

For maintaining the monomer content Cp in produced polymer particles atnot less than the reference monomer content C₁ in the polymer particlesat starting the polymerization, there are a method of sufficientlystirring the liquid phase to take the monomer from the gas phase, amethod of increasing the monomer content in the gas phase (increasing apressure of the gas phase) to forcedly supply the monomer to the liquidphase, a method of combining those methods, and the like.

When the volume of the gas phase is sufficiently present in the system,an area of an interface with the liquid phase increases and supplying ofthe monomer to the liquid phase and further to the polymer particles canbe conducted effectively.

In the preparation process of the present invention, a method ofcombining the stirring and the increasing of a monomer content(pressure) in the gas phase is preferred. This is because by thatmethod, the monomer content Cp in the polymer particles can bemaintained high throughout the polymerization. As mentioned above, it ispreferable to maintain the monomer content of the gas phase during thepolymerization at not less than 1.0 mole/liter, preferably not less than1.5 mole/liter, particularly preferably not less than 2.0 mole/liter.

As the means for stirring, there can be used, for example, an anchorblade, turbine blade, inclined blade, etc. From the viewpoint of goodmonomer diffusion and dispersion stability of the monomer, the stirringwith a large size blade called FULLZONE or MAXBLEND is preferred. Withrespect to a stirring equipment, either of horizontal stirrer orvertical stirrer may be employed. From the viewpoint of easy diffusionof the monomer, the horizontal stirrer is preferred.

The polymerization temperature is not limited particularly and anoptimum polymerization temperature is selected depending on kind of thepolymerization initiator. However if the polymerization temperature istoo high, there is a case where the monomer density in the gas phase iseasily lowered, a reaction for producing branch chains of the polymerarises and an intended polymer cannot be obtained. A preferablepolymerization temperature is from 40 to 120° C., more preferably from50° to 100° C.

The monomer may be supplied continuously or batchwise.

In the preparation process of the present invention, oil solubleperoxides can also be used as the polymerization initiator. Howeverthere are problems that the peroxides such asdi-isopropylperoxydicarbonate (IPP) and di-n-propylperoxydicarbonate(NPP) which are representative oil soluble polymerization initiatorsinvolve a danger of explosion, are expensive and easily cause cohesionof a scale on the wall of the tank during the polymerization. In thepresent invention, in order to further decrease a compression set, it ispreferable to use a water soluble polymerization initiator. Examples ofthe preferable water soluble polymerization initiator are, for instance,ammonium salt, potassium salt and sodium salt of persulfuric acid,perboric acid, perchloric acid, perphosphoric acid and percarbonic acid.Particularly preferred are ammonium persulfate and potassium persulfate.

An adding amount of the polymerization initiator is not limitedparticularly. An amount not lowering the polymerization rate remarkably(for example, several ppm based on water) or more may be added once atthe initial stage of the polymerization, intermittently or continuously.An upper limit of the adding amount is in the range in which apolymerization reaction heat can be eliminated from the surface of thereactor.

In the preparation process of the present invention, an emulsifyingagent and a molecular weight controller may be added. The molecularweight controller may be added once at the initial stage of thepolymerization, continuously or dividedly.

As the emulsifying agent, a nonionic surfactant, an anionic surfactantor a cationic surfactant can be used. Particularly fluorine-containingnonionic surfactants, for example, ammonium perfluorooctanoate arepreferred. An adding amount thereof (based on water as a polymerizationmedium) is preferably from 500 to 5,000 ppm.

Examples of the molecular weight controller are, for instance, esterssuch as dimethyl malonate, diethyl malonate, methyl acetate, ethylacetate, butyl acetate and dimethyl succinate and in addition,isopentane, isopropanol, acetone, various mercaptans, carbontetrachloride, cyclohexane, monoiodomethane, 1-iodomethane,1-iodo-n-propane, isopropyl iodide, diiodomethane, 1,2-diiodomethane,1,3-diiodo-n-propane, and the like.

In addition, a buffering agent, or the like may be added optionally inan amount not lowering the effect of the present invention.

The vulcanizable fluorine-containing elastomer composition of thepresent invention comprises the fluorine-containing elastomer,vulcanization agent and vulcanization aid.

The vulcanization agent which can be used in the present invention maybe optionally selected depending on an adopted vulcanization system. Asthe vulcanization system, there can be used any of a polyaminevulcanization system, polyol vulcanization system and peroxidevulcanization system. The effect of the present invention can beremarkably exhibited particularly when the vulcanization is carried outby the polyol vulcanization system.

Non-restricted examples of the vulcanization agent are, for instance,polyhydroxy compounds such as bisphenol AF, hydroquinone, bisphenol Aand diaminobisphenol AF in the polyol vulcanization system; forinstance, organic peroxides such asα-α′-bis(t-butylperoxy)diisopropylbenzene,2,5-dimethyl-2,5-di(t-butylperoxy)hexane and dicumyl peroxide in theperoxide vulcanization system; and, for instance, polyamine compoundssuch as hexamethylenediamine carbamate andN,N′-dicinnamylidene-1,6-hexamethylenediamine in the polyaminevulcanization system.

An adding amount of the vulcanization agent is from 0.01 to 10 parts byweight, preferably from 0.1 to 5 parts by weight based on 100 parts byweight of the elastomer.

As the vulcanization aid, there can be used organic base residues whichare usually used for the vulcanization of elastomer, such as variousquaternary ammonium salts, quaternary phosphonium salts, cyclic aminesand mono-functional amine compounds. Examples thereof are, for instance,quaternary ammonium salts such as tetrabutylammonium bromide,tetrabutylammonium chloride, benzyltributylammonium chloride,benzyltriethylammonium chloride, tetrabutylammoniumhydrogen sulfate andtetrabutylammonium hydroxide; quaternary phosphonium salts such asbenzyltriphenylphosphonium chloride, tributylallylphosphonium chloride,tributyl-2-methoxypropylphosphonium chloride andbenzylphenyl(dimethylamino)phosphonium chloride; mono-functional aminessuch as benzylmethylamine and benzylethanolamine; cyclic amines such as1,8-diazabicyclo[5.4.0]-undec-7-ene; and the like.

An adding amount of the vulcanization aid is from 0.01 to 10 parts byweight, preferably from 0.1 to 5.0 parts by weight based on 100 parts byweight of the elastomer.

Further there may be used usual additives such as a filler, processingaid, carbon black, inorganic filler, metal oxide such as magnesium oxideand metal hydroxide such as calcium hydroxide so far as the object ofthe present invention is not impaired.

The composition of the present invention is one satisfying theabove-mentioned equations (I), (II) and as the case demands, (III). Asfar as those conditions are satisfied, the preparation process andvulcanization method are not limited particularly, and usual processesand methods can be used.

Then the novel fluorine-containing elastomer of the present invention isexplained below.

The novel fluorine-containing elastomer of the present inventioncomprises VdF unit and HFP unit and is specified by the definition (A),that is:

-   (1) when a 0.2% by weight acetone solution of the    fluorine-containing elastomer is subjected to pressurized filtration    under a condition where acetone passes through at a flow rate F₀ of    2.4 g/min·cm², a ratio F₀/F₁ is from 1.2 to 3.5, provided that F₁ is    an amount of filtrate of the acetone solution during initial one    minute, and-   (2) a weight average molecular weight MW measured by GPC is from    50,000 to 600,000.

In the above-mentioned characteristic (1), the filtration amount ratioof acetone solution (namely, a ratio F₀/F₁ provided that a 0.2% byweight acetone solution of the fluorine-containing elastomer issubjected to pressurized filtration under a condition where acetonepasses through at a flow rate F₀ of 2.4 g/min-cm² and F₁ is an amount offiltrate of the acetone solution during initial one minute) is an indexindicating a degree of gelling of the fluorine-containing elastomer(hereinafter referred to as “gelling degree”). The smaller the gellingdegree is (namely, approximate to the flow (2.4 g/min·cm²) of acetonealone), the less the gelled molecules are. A gelling degree of known VdFelastomer is usually larger than 3.5. In the present invention, thegelling degree is preferably from 1.2 to 3.5 (actual amount of filtrateis from about 2 g/min·cm to about 0.7 g/min·cm²), particularlypreferably from 1.2 to 3.0.

In the above-mentioned characteristic (2), the weight average molecularweight MW measured by GPC is in the range capable of maintaining thecharacteristics of the elastomer, and a proper range is from 50,000 to600,000, preferably from 100,000 to 500,000.

In the novel fluorine-containing elastomer of the present invention, itis further preferable that (3) a ratio of an average molecular weightMW_(LS) measured by GPC-LALLS to MW_(RI) measured by GPC-RI, namely aratio MW_(LS)/MW_(RI) is from 1 to 2.5.

In the above (3), the ratio MW_(LS)/MW_(RI) of the average molecularweight MW_(LS) measured by GPC-LALLS to MW_(RI) measured by GPC-RI is anindex indicating a degree of branching of the molecule of the copolymer(hereinafter referred to as “branching coefficient”). When the ratio is1, it is defined that there is no branched chain substantially. Thehigher the ratio is, the more the number of branched chains increases.The fluorine-containing elastomer of the present invention has a lowbranching coefficient of from 1 to 2.5, which indicates that there aremany linear molecules. In the present invention it is particularlypreferable that the branching coefficient is from 1 to 1.5. The methodsof measuring the average molecular weight MW_(LS) with GPC-LALLS and theaverage molecular weight MW_(RI) with GPC-RI in the above (3) and themethod of measuring the weight average molecular weight MW with GPC inthe above (2) are explained hereinbelow.

The novel fluorine-containing elastomer of the present invention is afluorine-containing elastomer comprising VdF unit and HFP unit andhaving a weight average molecular weight MW measured by GPC of from50,000 to 600,000 and is specified by the definition (B) that when theelastomer is vulcanized in the above-mentioned standard formulationunder the above-mentioned standard vulcanization conditions, an optimumvulcanization time T₉₀ (min) and a compression set CS (%) satisfy thefollowing equations (I) and (II):

-   Equation (I): in case of 50,000≦MW≦200,000,    CS≦−3.45 Ln(T ₉₀)+22.8−4×10⁻⁵ ×MW-   Equation (II): in case of 200,000<MW,    CS≦−3.45 Ln(T ₉₀)+14.8.

Those equations are the same as those used for specifying theabove-mentioned fluorine-containing elastomer composition. In case ofthe above-mentioned invention of the composition, the equations make tosense in that the vulcanizable composition itself is specified. Howeverin this novel fluorine-containing elastomer, the elastomer itself isspecified by the equations, and the standard formulation and standardvulcanization conditions are specified since physical properties of theelastomer as a novel compound need to be specified. The above-mentionedstandard formulation and standard vulcanization conditions are usuallyused as formulation and vulcanization conditions of elastomers.

As the molecular weight increases, the compression set tends to belowered. Therefore when the weight average molecular weight is not lessthan 300,000, it is further preferable that the fluorine-containingelastomer composition satisfies the following equation (III).CS≦−3.45 Ln(T ₉₀)+13.3

The novel fluorine-containing elastomer (A) specified by the definition(A) and the novel fluorine-containing elastomer (B) specified by thedefinition (B) encompass some duplicates, but there are elastomers whichcan be specified only by either of the definitions.

From the viewpoint of decreasing the compression set, it is particularlypreferable that those fluorine-containing elastomers satisfy thefollowing equation (IV):Y≦−5.3 Ln(X)+28,provided that X is a Mooney viscosity (1+10) at 100° C. and Y is apercent by weight of a low molecular weight component having a weightaverage molecular weight measured by GPC of not more than 30,000 in thefluorine-containing elastomer.

The Mooney viscosity (1+10) at 100° C. is usually used as an index formold-processability. The Mooney viscosity in the range of from 20 to 150is usually considered allowable from the viewpoint of processability.However even if the Mooney viscosity is within the above-mentionedrange, when the low molecular weight component is contained in muchamount, the compression set of the obtained vulcanized articleincreases. Most of known fluorine-containing elastomers do not satisfythe above-mentioned equation (IV), and also even if the equation (IV) issatisfied, there are no elastomers satisfying the above-mentionedcharacteristics (1) to (3) and the equations (I) and (II)simultaneously.

Further it is preferable that a weight reduction ratio of thefluorine-containing elastomer of the present invention when heating at250° C. for 48 hours is not more than 1% by weight. If the weightreduction ratio is high, there is a case where thermal cracking arisesat heating for molding and evaporation of a low molecular weightcomponent occurs, thereby lowering quality of a molded article. A weightreduction ratio of known VdF elastomer usually exceeds 1% by weight, andtherefore very small voids are produced inside the elastomer during thevulcanization, which causes lowering of physical properties in normalstate of the obtained vulcanized article. Therefore in the presentinvention, it is preferable that the weight reduction ratio is as low aspossible, particularly not more than 0.35% by weight. When the weightreduction ratio is low, a molding temperature and vulcanizationtemperature can be made higher and there is a case where a secondaryvulcanization step can be omitted, which can enhance productivitysignificantly.

Those characteristics that any of branching coefficient, gelling degreeand weight reduction ratio are small indicate that thefluorine-containing elastomer of the present invention does not form agel, a content of a low molecular weight component is low, the number ofbranches are small and the molecules have a uniform structure.

Also as mentioned above, in conventional VdF elastomers, stabilizationof unstable end groups is carried out because if unstable groups (ionicgroups, etc.) are present in the molecular end, vulcanizability andmechanical properties are lowered. However in the fluorine-containingelastomer of the present invention having the above-mentionedcharacteristics, it was found that unexpectedly the presence of unstableend groups does not influence the mechanical properties of the elastomerat all and does not have any special effect on vulcanizationcharacteristics and on the contrary, the compression set tends to belowered. Therefore it becomes possible to increase an amount of thepolymerization initiator which causes generation of unstable end groups,and productivity of the fluorine-containing elastomer can be enhanced.Further introduction of ionic groups by positively reacting acidcomponents may be carried out from the viewpoint of decreasing thecompression set. From this point of view, it is preferable that thenovel fluorine-containing elastomer of the present invention containsfrom 10⁻⁵ to 10⁻² mole, preferably from 10⁻⁴ to 10⁻³ mole of ionic endgroups per 1 kg of the elastomer.

It is preferable that the fluorine-containing elastomers (A) and (B)contain from 50 to 90% by mole of VdF unit and 50 to 10% by mole of HFPunit, particularly preferably from 65 to 85% by mole of VdF unit and 35to 15% by mole of HFP unit.

The present invention further relates to the vulcanizablefluorine-containing elastomer composition comprising the above-mentionednovel fluorine-containing elastomer (A) or (B), a vulcanizing agent anda vulcanization aid. The vulcanizable fluorine-containing elastomercomposition itself comprising the above-mentioned novelfluorine-containing elastomer (A) or (B), a vulcanizing agent and avulcanization aid is not restricted by the above-mentioned equations (1)and (U). As the vulcanizing agent and vulcanization aid, those mentionedabove can be used.

The fluorine-containing elastomer composition and vulcanized article ofthe present invention have the following vulcanization characteristics,mold-processability, mechanical properties and compression set.Measuring methods thereof are explained hereinbelow.

[VdF Copolymer]

Vulcanization Characteristics (170° C.)

-   -   Minimum viscosity (ML): 0.098 to 2.94 (N)    -   Maximum viscosity (MH): 24.5 to 58.8 (N)    -   Induction time (T₁₀): 1.0 to 4.0 (min)    -   Optimum vulcanization time (T₉₀): 2.0 to 5.0 (min).        Mold-Processability    -   Contamination of a mold die does not occur and processability        with a roll is excellent.        [VdF Terpolymer]        Vulcanization Characteristics (170° C.)    -   Minimum viscosity (ML): 0.098 to 2.94 (N)    -   Maximum viscosity (MH): 24.5 to 58.8 (N)    -   Induction time (T₁₀): 1.5 to 5.0 (min)    -   Optimum vulcanization time (T₉₀): 2.5 to 6.5 (min).        Mold-Processability    -   Contamination of a mold die does not occur and processability        with a roll is excellent.

Also obtained vulcanized articles have the following physical propertiesand characteristics. Measuring methods thereof are explainedhereinbelow.

[VdF Copolymer]

Mechanical Properties (Physical Properties in Normal State)

-   -   100% modulus (M100): 1.96 to 8.82 (N/mm²)    -   Tensile strength at break (Tb): 9.8 to 19.6 (N/mm²)    -   Tensile elongation at break (Eb): 150 to 300 (%)    -   Hardness (Hs): 60 to 75.        Compression Set (CS)    -   After secondary vulcanization in oven: 7 to 12 (%)    -   After primary vulcanization by press: 20 to 35 (%).        [VdF Terpolymer]        Mechanic Properties (Physical Properties in Normal State)    -   100% modulus (M100): 1.96 to 7.87 (N/mm²)    -   Tensile strength at break (Tb): 14.7 to 24.5 (N/mm²)    -   Tensile elongation at break (Eb): 200 to 400 (%)    -   Hardness (Hs): 65 to 85.        Compression Set (CS)    -   After secondary vulcanization in oven: 10 to 17 (%)    -   After primary vulcanization by press: 25 to 45 (%).

The vulcanized article of the present invention can be applied on thefollowing molded articles, but the application is not limited to them.

Applications of Molded Articles:

Packing, O-ring, hose, other sealing material, diaphragm and valvehaving amine resistance, oil resistance, chemical resistance, steamresistance and weather resistance in transportation means such asautomotive, ship and airplane; similar packing, O-ring, sealingmaterial, diaphragm, valve, hose, roll and tube in chemical plant;similar packing, O-ring, hose, sealing material, belt, diaphragm, valve,roll and tube in foods plant and foods processing machine (includingdomestic appliances); similar packing, O-ring, hose, sealing material,diaphragm, valve and tube in nuclear plant; similar packing, O-ring,hose, sealing material, diaphragm, valve, roll, tube, mandrel, cable,flexible joint, belt, rubber plate and weatherstrip in generalindustrial parts; roll, roll blade and belt for PPC copying machine,etc.

More concretely there are the following applications.

(i) Automotive Applications

-   {circle around (1)} For sealing    -   Crank shaft seal    -   Gear box seal    -   Power piston packing    -   Cylinder liner seal    -   Valve stem seal    -   Front pump seal for automatic transmission    -   Rear axle pinion seal    -   Gasket for universal joint    -   Pinion seal for speed meter    -   Piston cup for foot brake    -   O-ring and oil seal for torque transmission    -   Seal for after-burner for exhaust gas    -   Bearing seal-   {circle around (2)} For hose    -   EGR tube    -   Twin-carburetor tube-   {circle around (3)} For diaphragm    -   Diaphragm for sensor of carburetor-   {circle around (4)} Other applications    -   Vibration proof rubber (engine mount, exhaust system, etc.)    -   Hose for after burner.        (ii) Applications in Chemical Industries-   {circle around (1)} For sealing    -   Seals for pump, flow meter and pipe for chemicals    -   Seal for heat exchanger    -   Packing of glass cooler for sulfuric acid manufacturing        equipment    -   Seals for sprinkler and transfer pump for agricultural chemicals    -   Seal for gas pipe    -   Seal for plating solution    -   Packing for high temperature vacuum dryer    -   Roll seal of belt for paper making    -   Seal for fuel battery    -   Joint seal for air duct-   {circle around (2)} For roll    -   Roll having trichlene resistance (for dyeing of fiber)-   {circle around (3)} Other applications    -   Acid resistant hose (for concentrated sulfuric acid)    -   Packing for joint of tubes for gas chromatograph and pH meter    -   Chlorine gas transfer hose    -   Rainwater drain hoses for benzene and toluene reservoir tanks    -   Seal, tube, diaphragm and valve parts for analyzer and physical        and chemical appliances.        (iii) Applications in Industrial Machinery-   {circle around (1)} For sealing    -   Seals for hydraulic and lubricating machine    -   Bearing seal    -   Seal for dry copying machine    -   Seals for window, etc. of dry cleaner    -   Seal for equipment for concentrating uranium hexafluoride    -   Seal (vacuum) valve for cyclotron    -   Seal for automatic packaging machine-   {circle around (2)} Other applications    -   Rolls, scraper, tube and valve parts for printing equipment    -   Rolls, scraper, tube and valve parts for coating equipment    -   Ink tube, roll and belt for printer    -   Belt and rolls for dry copying machine    -   Diaphragms for pumps for analyzing sulfurous acid gas and        chlorine gas in the air (environmental pollution-related meters)    -   Rolls and belt for printer    -   Squeeze rolls for pickling.        (iv) Airplane Applications    -   Valve stem seal for jet engine    -   Fuel feeding hose, gasket and O-ring    -   Rotating shaft seal    -   Gasket for hydraulic equipment    -   Seal for fire wall.        (v) Ship Applications    -   Stern seal for screw propeller shaft    -   Suction and exhaust valve stem seals for diesel engine    -   Valve seal for butterfly valve    -   Stem seal for butterfly valve.        (vi) Food and Medicine Applications    -   Seal for plate heat exchanger    -   Solenoid valve seal for vending machine    -   Plugs for chemicals.        (vii) Electrical Applications    -   Insulation oil cap for a train of the Shinkansen line    -   Venting seal for liquid ring transmission    -   Jacket for oil well cable.

Further the molded article of the present invention can be used forO-ring, sealing material, hose, tube, diaphragm, roll, lining andcoating in equipment for producing semiconductor, liquid crystal panel,plasma display panel, plasma address liquid crystal panel, fieldemission display panel, substrate for solar battery, etc., for example,CVD equipment, etching equipment, oxidation/diffusion equipment,sputtering equipment, ashing equipment, ion implantation equipment,exhausting equipment, and the like which require plasma resistance;O-ring, sealing material, hose, tube, diaphragm and roll in wet etcher,cleaning equipment, pipes for chemicals, gas pipes, and the like whichrequire chemical resistance; and further O-ring, sealing material, hose,tube, diaphragm and roll to be used on parts of the mentioned equipmentwhich are required to be free from dust and metal.

Examples of other applications where chemical resistance is required areO-ring, sealing material, hose, tube, diaphragm of pump and wafertransferring rolls for resist developing solution, releasing solutionand wafer cleaning solution in production equipment for semiconductor,liquid crystal panel, plasma display panel, etc.

The present invention is then explained concretely by means of examples,but is not limited thereto.

The methods of measuring the above-mentioned various properties andcharacteristics which are adopted in the present invention are asmentioned below.

[1] Gelling degree (F₀/F₁)

A amount of filtrate during a period of time of one minute from startingof the filtration is measured under the following filtration conditionsby using the following filter device.

-   -   Device (casing): KST-47 (available from ADVANTEC TOYO KABUSHIKI        KAISHA)    -   Effective filtration area: 47 mm diameter, 12.5 cm²    -   Kind of filter: PTFE membrane filter (available from ADVANTEC        TOYO KABUSHIKI KAISHA)    -   Opening: 0.1 μm    -   Size: 47 mm    -   Porosity: 68%    -   Filtration temperature: 25° C.    -   Filtration pressure: 0.2 kgf/cm²G    -   Solvent: Acetone    -   Sample: Pure acetone used as a solvent (Filtrate amount=F₀)        Acetone solution of 0.2% by weight of a polymer (Filtrate        amount=F₁).

A filtrate amount (passing amount) of pure acetone during a period ofinitial one minute per 1 cm² is 2.7 g/min·cm² under the above-mentionedconditions.

[2] Weight Average Molecular Weight MW

-   -   Device: HLC-8000 (available from TOSO KABUSHIKI KAISHA)    -   Column: Two columns TSK gel GMH_(XL)-H        -   One column TSK gel G3000H_(XL)        -   One column TSK gel G2000H_(XL)    -   Detector: Differential refraction meter    -   Developing solution: Tetrahydrofuran    -   Temperature: 35° C.    -   Sample concentration: 0.2% by weight    -   Standard sample: Various monodisperse polystyrenes ((Mw/Mn)=1.14        (Max)), TSK Standard POLYSTYRENE (available from TOSO KABUSHIKI        KAISHA).        [3] Low Molecular Weight Component Having a Molecular Weight of        not More than 30,000    -   Measured in the same manner as in the measurement of the weight        average molecular weight of above [2].        [4] Mooney Viscosity (1+10) 100° C.    -   Measured according to ASTM-D1646 and JIS K6300.    -   Measuring device: Automatic Mooney viscosity meter available        from KABUSHIKI KAISHA UESHIMA SEISAKUSHO    -   Number of rotations of rotor: 2 rpm    -   Measuring temperature: 100° C.        [5] Branching Coefficient (MW_(LS)/MW_(RI))

The average molecular weight (MW_(LS)) is calculated from a valueobtained by measuring with a light scattering photometer equipped withthe following GPC-LS device. The average molecular weight (MW_(RI)) iscalculated from a value obtained by measuring with a differentialrefraction meter and the same device. The measuring device and measuringconditions are described below.

-   -   Device: LS-8000 (available from TOSO KABUSHIKI KAISHA)    -   Column: TSK guard column H^(XL)-H        -   TSK gel G4000H_(XL)        -   TSK gel G3000H_(XL)        -   TSK gel GMH_(XL)-H        -   (one column each available from TOSO KABUSHIKI KAISHA)    -   Detector: Light scattering photometer for MW_(LS) (Wavelength of        light source 633 nm He—Ne)        -   Differential refraction meter for MW_(RI)    -   Developing solution: Tetrahydrofuran    -   Temperature: 35° C.    -   Sample concentration: 5 g/liter    -   Standard sample: Various monodisperse polystyrenes ((Mw/Mn)=1.14        (Max)), TSK Standard POLYSTYRENE (available from TOSO KABUSHIKI        KAISHA).        [6] Weight Reduction Ratio

300 ml of the aqueous dispersion of polymer obtained by thepolymerization reaction is continuously poured with stirring over oneminute into an aqueous solution of aluminum sulfate (obtained bydissolving 0.1 g of aluminum sulfate in pure water) previously put in ahigh speed stirring equipment to coagulate and separate the polymer.After removing water, 1,200 ml of clean water is put in, followed bystirring for two minutes to wash the polymer. After repeating thewashing operation three times, the polymer is removed and is dried in ahot air recirculation electric oven at 130° C. for 15 hours and then at150° C. for 3 hours to remove water. After allowing to stand for coolingto room temperature and then allowing to stand for one hour, the weightof the polymer is measured with a 5 g precise balance (W₀). Then thepolymer is heat-treated for 48 hours in the hot air recirculationelectric oven maintained at 250° C. and after allowing to stand forcooling to room temperature and then allowing to stand for one hour, theweight of the polymer is measured with a precise balance (W₁).

A value obtained by dividing the weight reduction (ΔW) after the heattreatment by the weight (We) before the heat treatment is the weightreduction ratio (% by weight).

[7] Content of Ionic End Group

Since the ionic end groups are produced by the deterioration of thepolymerization initiator, the content of the ionic end groups (Number ofionic groups per 1 kg of polymer=mole/1 kg of polymer) is calculated bythe following equation.Content of ionic end groups (mole/1 kg of polymer)=2×f×I ₀(1−e^(−kdt))/W×10³in which

-   f: Efficiency of polymerization initiator (assumed to be f=0.5 at    80° C.)-   I₀: Adding amount of polymerization initiator (mmol)-   kd: Constant of deterioration speed (1/min) (assumed to be 0.0057 at    80° C. in case of APS)-   t: Polymerization time (min)-   W: Weight of obtained polymer (kg).    [8] Vulcanization Characteristics    (Vulcanization Method)

The polymer (elastomer) is subjected to polyol vulcanization in thestandard formulation under standard vulcanization conditions.

Standard Formulation

-   -   Elastomer; 100 parts by weight    -   Vulcanizing agent: 2.2 parts by weight of bisphenol AF    -   Vulcanization aid: 0.4 part by weight of        benzyltriphenylphosphonium chloride    -   Additive: 3 parts by weight of highly active magnesium oxide        -   20 parts by weight of carbon black (MT-C)        -   6 parts by weight of calcium hydroxide.            Standard Vulcanization Conditions    -   Kneading method: Kneading by roll    -   Vulcanization by press: at 170° C. for 10 minutes    -   Vulcanization in an oven: at 230° C. for 24 hours.

A vulcanization curve at 170° C. is obtained at the primaryvulcanization by press with a JSR Curastometer model II by theabove-mentioned vulcanization method and then a minimum viscosity (ML),maximum viscosity (MH), induction time (T₁₀) and optimum vulcanizationtime (T₉₀) are obtained.

[9] Stability in Molding

The components of the above-mentioned standard formulation are subjectedto press molding at 180° C. for 3 minutes with P-8 mold die for O-ring(for 65 rings). The molding is carried out under the conditions easilycausing molding failure, namely under the conditions that amold-releasing agent is not used and deairing is not carried out.

With respect to O-rings after the molding, the stability in molding isevaluated by the number of O-rings in which there are recognized failurein fusion, generation of voids or contamination of the mold die.

[10] Processability by Roll

The components of the above-mentioned standard formulation are subjectedto processing with a kneading roll set at a thickness of 1 mm. Theprocessability at kneading by roll such as roll mixing processabilityand sheeting is evaluated with naked eyes.

[11] Mechanical Properties

Physical Properties in Normal State

The components of the above-mentioned standard formulation are subjectedto primary press vulcanization and secondary oven vulcanization underthe above-mentioned standard vulcanization conditions to make a 2 mmthick sheet. 100% modulus (M100), tensile strength at break (Tb) andtensile elongation at break (Eb) are measured according to, JIS-K6251and a hardness (Hs) is measured according to JIS-K6253.

Compression Set

The components having the above-mentioned standard formulation aresubjected to primary press vulcanization and secondary ovenvulcanization under the above-mentioned standard vulcanizationconditions to make O-ring (P-24). According to JIS-K6301, compressionset after the primary press vulcanization and compression set (CS) afterthe secondary oven vulcanization (Measuring is carried out using asample subjected to holding at 200° C. for 70 hours under 25%compression and then allowing to stand for 30 minutes in a 25° C.constant temperature chamber) are measured.

PREPARATION EXAMPLE 1

(Preparation of Seed Polymer Particles)

A 1.8 liter polymerization tank equipped with an electromagnetic stirreras a stirring device was charged with 720 g of pure water, 290 g of anaqueous solution of 10% by weight of ammonium perfluorooctanoate and 0.6g of diethyl malonate. After the inside of a system was sufficientlyreplaced with nitrogen gas, the inside pressure was decreased. Thosereplacing and pressure reducing operations were repeated three times,and 20 g of VdF and 51 g of HFP were introduced under reduced pressure,followed by heating up to 80° C. with stirring. Then 0.6 g of ammoniumpersulfate (APS) dissolved in 0.02 g of pure water was introduced withpressurized nitrogen gas to initiate the polymerization. Thepolymerization pressure was 2 MPa. In order to make up for decrease ofpressure during the polymerization, a VdF/HFP monomer mixture (78/22% bymole) was supplied continuously. The polymerization was carried out withstirring. Thirty minutes after, the stirring was stopped and themonomers were released to terminate the polymerization.

The weight of the obtained polymer particles was 200 g, and the weightaverage molecular weight measured by the above-mentioned method was280,000. The components of the polymer measured according to ¹⁹F-NMR wasVdF/HFP=76.6/23.4 (% by mole).

(Preparation of Fluorine-Containing Elastomer)

The same 1.8 liter polymerization tank equipped with a stirrer as abovewas charged with 950 g of pure water, 18 g of the aqueous dispersion(concentration: 16.6% by weight) of polymer particles prepared above,4.0 g of diethyl malonate and 14.7 g of an aqueous solution of 10. % byweight of ammonium perfluorooctanoate. After the inside of a system wassufficiently replaced with nitrogen gas, the inside pressure wasdecreased. Those replacing and pressure reducing operations wererepeated three times, and 125 g of VdF and 450 g of HFP were introducedunder reduced pressure, followed by heating up to 80° C. with stirring.Then 0.05 g of APS dissolved in 20 g of pure water was introduced withpressurized nitrogen gas to initiate the polymerization. Thepolymerization was continued under the following conditions and 7 hoursafter, the stirring was stopped and the monomers were released toterminate the polymerization.

-   -   (a) In order to maintain the total content of the monomers in        the gas phase at 6 mole/liter, the VdF/HFP (78/22% by mole)        monomer mixture was continuously supplied and the pressure in        the gas phase was maintained at 6 MPa (absolute pressure).    -   (b) The stirring speed was maintained at 700 rpm.

When the stirring speed was changed under the polymerization conditionsof this Preparation Example, the polymerization rate was constant at thestirring speed of not less than 290 rpm. From this point of view, underthe above-mentioned conditions, the monomer content Cp in the polymerwas higher than C₁.

The weight of the obtained fluorine-containing elastomer was 320 g, andthe weight average molecular weight MW measured by the above-mentionedmethod was 220,000. The components of the polymer measured according to¹⁹F-NMR was VdF/HFP=77.7/22.3 (% by mole).

Further with respect to this fluorine-containing elastomer, a branchingcoefficient, a gelling degree, a content of low molecular weightcomponent having a molecular weight of not more than 30,000, a Mooneyviscosity, a weight reduction ratio and a content of ionic end groupswere measured according to the above-mentioned methods. The results areshown in Table 1.

PREPARATION EXAMPLES 2 TO 6

The fluorine-containing elastomer was prepared in the same manner as inPreparation Example 1 using the polymer particles prepared inPreparation Example 1 except that the amounts of APS and diethylmalonate and the polymerization conditions were changed as shown inTable 1. Properties of the obtained fluorine-containing elastomer areshown in Table 1.

The stirring speeds of each Preparation Example where the polymerizationrate became constant were 330 rpm in Preparation Example 2, 400 rpm inPreparation Example 3, 450 rpm in Preparation Example 4, 390 rpm inPreparation Example 5 and 550 rpm in Preparation Example 6. From thispoint of view, under the polymerization conditions shown in Table 1, anyof the monomer contents Cp in the polymer were higher than C₁.

COMPARATIVE PREPARATION EXAMPLE 1

The same 1.8 liter polymerization tank equipped with a stirrer as inPreparation Example 1 was charged with 970 g of pure water and 18 g ofan aqueous dispersion (concentration: 16.7% by weight) of seed polymerparticles prepared above. After the inside of a system was sufficientlyreplaced with nitrogen gas, the inside pressure was decreased. Thosereplacing and pressure reducing operations were repeated three times,and 15 g of VdF and 24 g of HFP were introduced under reduced pressure,followed by heating up to 60° C. with stirring. Then 1.7 g ofdiisopropylperoxydicarbonate (TIP) dissolved in 14.7 g of an aqueoussolution of 10% by weight of ammonium perfluorooctanoate was introducedwith pressurized nitrogen gas to initiate the polymerization. Thepolymerization pressure was adjusted to 1.5 MPa. In order to make up fordecrease of pressure during the polymerization, the VdF/HFP (78/22% bymole) monomer mixture was continuously supplied, and the polymerizationwas carried out with stirring (The stirring conditions were the same asin Preparation Example 1). Five hours after, the stirring was stoppedand the monomers were released to terminate the polymerization.

The polymerization conditions were as follows.

-   -   (a) Total content of the monomers in the gas phase: 0.5        mole/liter (The pressure in the gas phase was maintained at 1.5        MPa (absolute pressure)).    -   (b) Stirring speed: 400 rpm.

In the polymerization system of Comparative Preparation Example 1, thepolymerization rate continued to increase even at the stirring speed of800 rpm.

The weight of the obtained fluorine-containing elastomer for comparisonwas 370 g, and the weight average molecular weight MW measured by theabove-mentioned method was 220,000. The components of the polymermeasured according to ¹⁹F-NMR was VdF/HFP=77.7/22.3 (% by mole).

Further with respect to this fluorine-containing elastomer forcomparison, a branching coefficient, a gelling degree, a content of lowmolecular weight component having a molecular weight of not more than30,000, a Mooney viscosity, a weight reduction ratio and a content ofionic end groups were measured according to the above-mentioned methods.The results are shown in Table 1.

COMPARATIVE PREPARATION EXAMPLE 2

The same 1.8 liter polymerization tank equipped with a stirrer as inPreparation Example 1 was charged with 950 g of pure water. After theinside of a system was sufficiently replaced with nitrogen gas, theinside pressure was decreased. Those replacing and pressure reducingoperations were repeated three times, and 12 g of VdF and 20 g of HFPwere introduced under reduced pressure, followed by heating up to 80° C.with stirring. Then 0.064 ml of isopentane was introduced and further0.6 g of APS dissolved in 10 g of pure water was introduced withpressurized nitrogen gas to initiate the polymerization. Thepolymerization pressure was adjusted to 1.2 MPa. The introduction of0.064 ml of isopentane and the introduction of 0.6 g of APS dissolved in10 g of pure water under pressure were carried out every 50 minutesuntil the completion of the polymerization. In order to make up fordecrease of pressure during the polymerization, the VdF/HFP (78/22% bymole) monomer mixture was continuously supplied, and the polymerizationwas carried out with stifling (The stirring conditions were the same asin Preparation Example 1). 200 minutes after, the stirring was stoppedand the monomers were released to terminate the polymerization.

-   -   (a) Total content of the monomers in the gas phase: 0.4        mole/liter (The pressure in the gas phase was maintained at 1.2        MPa (absolute pressure)).    -   (b) Stirring speed: 400 rpm.

In the polymerization system of Comparative. Preparation Example 2, thepolymerization rate continued to increase even at the stirring speed of800 rpm.

The weight of the obtained fluorine-containing elastomer for comparisonwas 370 g, and the weight average molecular weight MW measured by theabove-mentioned method was 190,000. The components of the polymermeasured according to ¹⁹F-NMR was VdF/HFP=78.5/21.5 (% by mole).

Further with respect to this fluorine-containing elastomer forcomparison, a branching coefficient, a gelling degree, a content of lowmolecular weight component having a molecular weight of not more than30,000, a Mooney viscosity, a weight reduction ratio and a content ofionic end groups were measured according to the above-mentioned methods.The results are shown in Table 1.

TABLE 1 Com. Com. Prep. Prep. Prep. Prep. Prep. Prep. Prep. Prep. Ex. 1Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 1 Ex. 2 Polymerization conditionsCharged monomer (% by mole) VdF 40 40 40 40 40 40 58 60 HFP 60 60 60 6060 60 42 40 Solvent (g) Pure water 950 950 950 950 950 950 970 950Additive Diethyl malonate (g) 4.0 4.0 4.3 3.3 6.5 2.0 — — Isopentane(ml) — — — — — — — 0.26 Polymerization initiator (g) APS 0.05 0.1 0.30.3 0.3 0.4 — 2.1 IIP — — — — — — 1.7 — Polymerization temperature (°C.) 80 80 80 80 80 80 60 80 Polymerization time (min) 420 180 85 60 12070 300 200 Polymerization conditions Content of monomer in the gas phase6 6 6 6 6 2.5 0.5 0.4 (mole/liter) Pressure in the gas phase (MPa) 6 6 66 6 4 1.5 1.2 Stirring speed (rpm) 700 700 700 700 700 800 400 400Properties of polymer Components (% by mole) VdF 77.7 78.4 78.1 79.077.6 76.5 77.7 78.5 HFP 22.3 21.6 21.9 21.0 22.4 23.5 22.3 21.5 Weightaverage molecular weight MW (×10⁵) 2.2 2.5 2.2 3.1 1.5 2.5 2.2 1.9Branching coefficient (MW_(LS)/MW_(RI)) 1.14 1.25 1.13 1.18 1.09 1.121.48 1.52 Gelling degree (F₀/F₁) 2.26 2.45 2.42 2.67 1.83 2.51 5.21 5.71Mooney viscosity (1 + 10) 100° C. 68 70 64 100 41 74 68 59 Content oflow molecular weight component 5.6 5.5 6.0 3.6 8.3 5.2 5.6 6.4 having amolecular weight of not more than 30,000 which was calculated byequation (IV) (% by weight) Measured content of low molecular weight 4.14.4 4.3 2.5 6.0 5.2 6.4 6.8 component having a molecular weight of notmore than 30,000 (% by weight) Weight reduction ratio (% by weight) 0.170.19 0.19 0.15 0.26 0.40 0.40 0.74 Content of ionic end groups 0.62 0.811.33 1.11 1.03 1.41 0.11 12.1 (mole/1 kg of polymer × 10⁻³)

EXAMPLES 1 TO 6 AND COMPARATIVE EXAMPLES 1 TO 2

Elastomer compositions for vulcanization were prepared using thefluorine-containing elastomers obtained in Preparation Examples 1 to 6and Comparative Preparation Examples 1 to 2, respectively with theabove-mentioned standard formulation, and vulcanizability, stability inmolding and processability by roll of the compositions were evaluated bythe above-mentioned methods. Further vulcanizability, physicalproperties in normal state and compression set of vulcanized articleswere evaluated by the above-mentioned methods. The results are shown inTable 2.

TABLE 2 Com. Com. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 1 Ex. 2Vulcanizability (170° C.) ML (kgf) 0.13 0.12 0.11 0.16 0.05 0.23 0.180.18 MH (kgf) 5.20 4.50 4.60 5.30 2.80 5.00 4.60 4.10 T₁₀ (min) 2.002.00 2.50 2.40 2.80 3.00 2.10 4.30 T₉₀ (min) 2.50 2.80 3.00 3.10 3.404.10 2.80 6.10 Stability in molding 1 1 2 1 2 3 5 11 Processability byroll ◯ ◯ ◯ ◯ ◯ ◯ Δ ◯ Physical properties in normal state M100 (N/mm²)5.3 4.8 5.1 5.3 5.5 4.4 4.7 5.1 Tb (N/mm²) 14.4 14.7 14.9 14.0 15.8 15.614.1 13.9 Eb (%) 230 230 230 210 230 258 240 200 Hs (JIS) 69 69 68 69 7070 70 69 Compression set (CS) (%) After primary press vulcanization — 2627 — — — 47 32 After secondary oven vulcanization 10 10 9 8 12 9 13 11

PREPARATION EXAMPLES 7 TO 9

Fluorine-containing elastomers were prepared in the same manner as inPreparation Example 1 using the polymer particles prepared inPreparation Example 1 except that the polymerization temperature, addingmethod and kind of the molecular weight controller, etc. were changed asshown in Table 3. Properties of the obtained fluorine-containingelastomers are shown in Table 3.

The stirring speeds of each Preparation Example where the polymerizationrate became constant were 500 rpm in Preparation Example 7, 450 rpm inPreparation Example 8 and 400 rpm in Preparation Example 9. From thispoint of view, under the polymerization conditions shown in Table 3, anyof the monomer contents Cp in the polymer were higher than C₁.

TABLE 3 Prep. Prep. Prep Ex. 7 Ex. 8 Ex. 9 Polymerization conditionsCharged monomer (% by mole) VdF 42 41 42 HFP 58 59 58 Solvent (g) Purewater 960 960 960 Additive Diethyl malonate (g) 3.0 0.3 g was — addedinitially and 12 g was added when an amount of the obtained polymerreached 150 g. Isopentane (ml) — — 1.84 Polymerization initiator (g) 0.30.1 0.3 APS Polymerization temperature (° C.) 70 80 80 Polymerizationtime (min) 180 166 220 Polymerization conditions Content of monomer inthe gas phase 6 6 6 (mole/liter) Pressure in the gas phase (MPa) 4.6 6 6Stirring speed (rpm) 700 700 700 Properties of polymer Components (% bymole) VdF 78.0 77.8 79.0 HFP 22.0 22.2 21.0 Weight average molecularweight 2.8 4.0 2.2 MW (×10⁵) Branching coefficient (MW_(LS)/MW_(RI))1.20 1.8 1.15 Gelling degree (F₀/F₁) 2.18 2.3 2.15 Mooney viscosity (1 +10) 100° C. 101 66 65 Content of low molecular weight 3.5 5.8 5.9component having a molecular weight of not more than 30,000 which wascalculated by equation (IV) (% by weight) Measured content of lowmolecular weight 3.0 5.6 3.9 component having a molecular weight of notmore than 30,000 (% by weight) Weight reduction ratio (% by weight) 0.210.22 0.11 Content of ionic end groups 2.2 0.7 2.5 (mole/1 kg of polymer× 10⁻³)

EXAMPLES 7 TO 9

Elastomer compositions for vulcanization were prepared using thefluorine-containing elastomers obtained in Preparation Examples 7 to 9,respectively with the above-mentioned standard formulation, andvulcanizability, stability in molding and processability by roll of thecompositions were evaluated by the above-mentioned methods. Furthervulcanizability, physical properties in normal state and compression setof vulcanized articles were evaluated by the above-mentioned methods.The results are shown in Table 4.

TABLE 4 Ex. 7 Ex. 8 Ex. 9 Vulcanizability (170° C.) ML (kgf) 0.22 0.230.14 MH (kgf) 5.03 4.6 4.72 T₁₀ (min) 2.1 2.3 2.5 T₉₀ (min) 3.0 3.3 3.0Stability in molding 3 2 5 Processability by roll ◯ ◯ ◯ Physicalproperties in normal state M100 (N/mm²) 4.8 4.5 5.3 Tb (N/mm²) 14.7 12.514.2 Eb (%) 240 225 210 Hs (JIS) 71 69 69 Compression set (CS) (%) Afterprimary press vulcanization 28 29 27 After secondary oven vulcanization9.2 10.2 9.0

INDUSTRIAL APPLICABILITY

According to the present invention, a vulcanized article having a smallcompression set can be produced in a short period of time and avulcanizable fluorine-containing elastomer composition having excellentstability in molding and kneading property can be provided. Thevulcanized article is suitable particularly as a sealing material.

The present invention can also provide a fluorine-containing elastomerin which branched chains and gelled portion are reduced and a content ofa low molecular weight component is small.

1. A fluorine-containing elastomer which comprises vinylidene fluorideunit and hexafluoropropylene unit and is characterized in that: (1) whena 0.2% by weight acetone solution of the fluorine-containing elastomeris subjected to pressurized filtration under a condition where acetonepasses through at a flow rate F₀ of 2.4 g/min·cm², a ratio F₀/F₁ is from1.2 to 3.5, provided that F₁ is an amount of filtrate of the acetonesolution during initial one minute, and (2) a weight average molecularweight MW measured by GPC is from 50,000 to 600,000.
 2. Thefluorine-containing elastomer of claim 1, wherein (3) a ratio ofMW_(LS)/MW_(RI) of an average molecular weight MW_(LS) measured byGPC-LALLS to MW_(RI) measured by GPC-RI is from 1 to 2.5.
 3. Thefluorine-containing elastomer of claim 1, wherein said ratio F₀/F₁ ofthe fluorine-containing elastomer is from 1.2 to 3.0.
 4. Afluorine-containing elastomer which comprises vinylidene fluoride unitand hexafluoropropylene unit and is characterized in that a weightaverage molecular weight MW measured by GPC is from 50,000 to 600,000,and when the elastomer is vulcanized in a standard formulation understandard vulcanization conditions, an optimum vulcanization time T₉₀(min) and a compression set CS (%) satisfy the following equations (I)and (II): Equation (I): in case of 50,000≦MW≦200,000,CS≦−3.45 Ln(T ₉₀)+22.8−4×10⁻⁵ ×MW Equation (II): in case of 200,000<MW,CS≦−3.45 Ln(T ₉₀)+14.8.
 5. The fluorine-containing elastomer of claim 1,wherein the elastomer satisfies the following equation (IV):Y<−5.3 Ln(X)+28, provided that X is a Mooney viscosity (1+10) at 100° C.of the fluorine-containing elastomer and Y is a percent by weight of alow molecular weight component having a weight average molecular weightmeasured by GPC of not more than 30,000 in the fluorine-containingelastomer.
 6. The fluorine-containing elastomer of claim 1, wherein aweight reduction ratio of the elastomer when heated at 250° C. for 48hours is not more than 1% by weight.
 7. The fluorine-containingelastomer of claim 1, which contains ionic end groups in an amount offrom 10⁻⁵ to 10⁻² mole per 1 kg of the elastomer.
 8. Thefluorine-containing elastomer of claim 1, which comprises 50 to 90% bymole of the vinylidene fluoride unit and 50 to 10% by mole of thehexafluoropropylene unit.
 9. A vulcanizable fluorine-containingelastomer composition which comprises a vulcanization agent, avulcanization aid and a fluorine-containing elastomer having C—H bond inits polymer chain and characterized in that: (1) when a 0.2% by weightacetone solution of the fluorine-containing elastomer is subjected topressurized filtration under a condition where acetone passes through ata flow rate F₀ of 2.4 g/min·cm², a ratio F₀/F₁ is from 1.2 to 3.5,provided that F₁ is an amount of filtrate of the acetone solution duringinitial one minute, and (2) a weight average molecular weight MWmeasured by GPC is from 50,000 to 600,000.
 10. The fluorine-containingelastomer composition of claim 9, wherein the fluorine-containingelastomer is characterized in that: (3) a ratio of MW_(LS)/MW_(RI) of anaverage molecular weight MW_(LS) measured by GPC-LALLS, to MW_(RI)measured by GPC-RI is from 1 to 2.5.
 11. A vulcanizablefluorine-containing elastomer composition which comprises avulcanization agent, a vulcanization aid and a fluorine-containingelastomer having C—H bond in its polymer chain and characterized in thata weight average molecular weight MW measured by GPC of the elastomer isfrom 50,000 to 600,000 and when the elastomer is vulcanized in astandard formulation under standard vulcanization conditions, an optimumvulcanization time T₉₀ (min) and a compression set CS (%) of theelastomer satisfy the following equations (I) and (I): Equation (I): incase of 50,000≦MW≦200,000,CS≦−3.45 Ln(T ₉₀)+22.8−4×10⁵ ×MW Equation (II): in case of 200,000<MW,CS≦−3.45 Ln(T ₉₀)+14.8.
 12. A vulcanizable fluorine-containing elastomercomposition comprising a vulcanization agent, a vulcanization aid and afluorine-containing elastomer having C—H bond in its polymer chain,wherein an optimum vulcanization time T₉₀ (min) of the composition and acompression set CS (%) of a vulcanized article satisfy the followingequations (I) and (II): Equation (I): in case of 50,000≦MW≦200,000,CS≦−3.45 LN(T ₉₀)+22.8−4×10⁵ ×MW Equation (II): in case of 20,000<MW,CS≦−3.45 LN(T ₉₀)+14.8.
 13. The composition of claim 9, wherein saidfluorine-containing elastomer is a copolymer of vinylidene fluoride andother monomer.
 14. The composition of claim 13, wherein said othermonomer is at least one selected from the group consisting ofhexafluoropropylene, tetrafluoroethylene and perfluoro(alkyl vinylether).
 15. The composition of claim 9, wherein said vulcanization agentis at least one selected from the group consisting of polyhydroxycompounds, polyamine compounds and organic peroxides.
 16. Thecomposition of claim 15, wherein said polyhydroxy compound is at leastone selected from the group consisting of bisphenol AF, hydroquinone,bisphenol A and diaminobisphenol AF.
 17. The composition of claim 15,wherein said organic peroxide is at least one selected from the groupconsisting of α,α′-bis(t-butylperoxy)diisopropylbenzene,2,5-dimethyl-2,5-di(t-butylperoxy)hexane and dicumyl peroxide.
 18. Thecomposition of claim 15, wherein said polyamine compound ishexamethylenediamine carbamate and/orN,N′-dicinnamylidene-1,6-hexamethylenediamine.
 19. The composition ofclaim 9, wherein said vulcanization aid is at least one organic baseresidue selected from the group consisting of quaternary ammonium salts,quaternary phosphonium salts, cyclic amines and mono-functional aminecompounds.
 20. A vulcanized article obtained by vulcanizing thefluorine-containing elastomer composition of claim
 9. 21. A process forpreparing a fluorine-containing elastomer having C—H bond in its polymerchain by emulsion-polymerizing polymerizable monomers in an aqueousmedium in a reactor, in which the polymerization is carried out underthe conditions that: (a) a reaction system in the reactor comprises agas phase and a liquid phase, and (b) during the polymerizationreaction, a content Cp of the polymerizable monomers in the polymerparticles which are in the process of polymerization reaction ismaintained at a reference content C₁ or more of the polymerizablemonomers in the polymer particles.
 22. The preparation process of claim21, wherein the content Cp of the polymerizable monomers in the polymerparticles which are in the process of polymerization reaction ismaintained at a reference content C₂ or more of the polymerizablemonomers in the polymer particles.
 23. The preparation process of claim21, wherein the polymerizable monomers are supplied into the reactorcontinuously or batchwise.
 24. The preparation process of claim 21,wherein a water soluble radical polymerization initiator is used. 25.The preparation process of claim 21, wherein the polymerizable monomersare vinylidene fluoride and other monomer.
 26. The preparation processof claim 25, wherein said other monomer is at least one selected fromthe group consisting of hexafluoropropylene, tetrafluoroethylene andperfluoro(alkyl vinyl ether).
 27. The fluorine-containing elastomer ofclaim 4, wherein the elastomer satisfies the following equation (IV):Y<−5.3 Ln(X)+28, provided that X is a Mooney viscosity (1+10) at 100° C.of the fluorine-containing elastomer and Y is a percent by weight of alow molecular weight component having a weight average molecular weightmeasured by GPC of not more than 30,000 in the fluorine-containingelastomer.
 28. The fluorine-containing elastomer of claim 4, wherein aweight reduction ratio of the elastomer when heated at 250° C. for 48hours is not more than 1% by weight.
 29. The fluorine-containingelastomer of claim 4, which contains ionic end groups in an amount offrom 10⁻⁵ to 10⁻² mole per 1 kg of the elastomer.
 30. Thefluorine-containing elastomer of claim 4, which comprises 50 to 90% bymole of the vinylidene fluoride unit and 50 to 10% by mole of thehexafluoropropylene unit.
 31. The composition of claim 11, wherein saidfluorine-containing elastomer is a copolymer of vinylidene fluoride andother monomer.
 32. The composition of claim 31, wherein said othermonomer is at least one selected from the group consisting ofhexafluoropropylene, tetrafluoroethylene and perfluoro(alkyl vinylether).
 33. The composition of claim 11, wherein said vulcanizationagent is at least one selected from the group consisting of polyhydroxycompounds, polyamine compounds and organic peroxides.
 34. Thecomposition of claim 33, wherein said polyhydroxy compound is at leastone selected from the group consisting of bisphenol AF, hydroquinone,bisphenol A and diaminobisphenol AF.
 35. The composition of claim 33,wherein said organic peroxide is at least one selected from the groupconsisting of α,α′-bis(t-butylperoxy)diisopropylbenzene,2,5-dimethyl-2,5-di(t-butylperoxy)hexane and dicumyl peroxide.
 36. Thecomposition of claim 33, wherein said polyamine compound ishexamethylenediamine carbamate and/orN,N′-dicinnamylidene-1,6-hexamethylenediamine.
 37. The composition ofclaim 11, wherein said vulcanization aid is at least one organic baseresidue selected from the group consisting of quaternary ammonium salts,quaternary phosphonium salts, cyclic amines and mono-functional aminecompounds.
 38. A vulcanized article obtained by vulcanizing thefluorine-containing elastomer composition of claim
 11. 39. Thecomposition of claim 12, wherein said fluorine-containing elastomer is acopolymer of vinylidene fluoride and other monomer.
 40. The compositionof claim 39, wherein said other monomer is at least one selected fromthe group consisting of hexafluoropropylene, tetrafluoroethylene andperfluoro(alkyl vinyl ether).
 41. The composition of claim 12, whereinsaid vulcanization agent is at least one selected from the groupconsisting of polyhydroxy compounds, polyamine compounds and organicperoxides.
 42. The composition of claim 41, wherein said polyhydroxycompound is at least one selected from the group consisting of bisphenolAF, hydroquinone, bisphenol A and diaminobisphenol AF.
 43. Thecomposition of claim 41, wherein said organic peroxide is at least oneselected from the group consisting ofα,α′-bis(t-butylperoxy)diisopropylbenzene,2,5-dimethyl-2,5-di(t-butylperoxy)hexane and dicumyl peroxide.
 44. Thecomposition of claim 41, wherein said polyamine compound ishexamethylenediamine carbamate and/orN,N′-dicinnamylidene-1,6-hexamethylenediamine.
 45. The composition ofclaim 12, wherein said vulcanization aid is at least one organic baseresidue selected from the group consisting of quaternary ammonium salts,quaternary phosphonium salts, cyclic amines and mono-functional aminecompounds.
 46. A vulcanized article obtained by vulcanizing thefluorine-containing elastomer composition of claim 12.